Protective system for electro-erosion metalworking



April 15, 1969 w. ULLMANN ET AL 3,439,146

PROTECTIVE SYSTEM FOR ELECTRO-EROSION METALWORKING Filed June 27, 1966 United States Patent 3,439,146 PROTECTIVE SYSTEM FOR ELECTRO-EROSION METALWORKING Werner Ullmann and Franco Donati, Locarno, Switzerland, assignors to A.G. fur industrielle Elektronik AGIE Losone bei Locarno, Losone-Locarno, Switzerland, a Swiss corporation Filed June 27, 1966, Ser. No. 560,503 Claims priority, application Switzerland, July 15, 1965, 9,968/65 Int. Cl. B23k 9/10 US. Cl. 219-69 13 Claims The present invention relates to an improved protective device or arrangement for electro-erosive metal workinging cooperating with a pulse generator and a reversible electrode feed mechanism for the interruption of arcs in the working gap.

In electro-erosi-ve metalworking, undesirable arcing often occurs in the work gap (between the workpiece and the work electrode). These arc-like discharges occur, for example, due to uncontrollable reduction and soiling of the work gap during operation. Reduction of the work gap may be caused by decomposition of the dielectric liquid by the working pulses, whereby a kind of soot is formed, which becomes firmly deposited in the gap. Normally, these impurities are washed away by the dielectric liquid. However, practice has shown that, for example, in electro-erosive rough working (roughing) of workpieces, in which a very large removal of material from the workpiece surface is required within an extremely short time, and/ or when pulse generators having no storage means are employed, which produce working sparks of great energy density, these impurities cannot be washed away by the dielectric liquid. The impurities produce an artificial reduction of the working gap and thus a seat for the formation of arcs.

The harmful nature of arcs in electro-erosive metalworking is well known. The arc produces a change in structure in the surface of the workpiece to be worked to a depth of millimeters. Additionally, the shape of the work electrode is not accurately reproduced in the workpiece because the arc melts parts of the surface.

In known reversible feed arrangements the work electrode is withdrawn from the work gap upon occurrence of an are. It is thereby intended to interrupt the arc, but the arc is pulled along with the electrode when the latter is withdrawn, especially when high energy density is employed. Therefore, safety devices have already been proposed in combination with reversible feed arrangements in which, immediately when an arcing tendency occurs in the work gap, the current supply to the electro-erosion circuit is interrupted. These safety devices, however, respond much too often owing to their over-sensiti'veness and interrupt the electro-erosive working operation too frequently, so that despite good pulse generators, the efliciency of the whole installation is lowered.

Other safety installations respond when the voltage in the work gap falls below a particular value. However, practice has shown that the arc voltage, integrated over time, may sometimes have a higher value than the integrated voltage of the individual working pulses. Therefore, in these cases the safety devices do not respond to arcs, with the result that the arc can continue to burn for a very long time although the reversible electrode feed arrangement withdraws the electrode from the work gap.

Accordingly, it is a primary object of the present invention to provide an improved protective device for electro-erosion metalworking which effectively overcomes the aforementioned drawbacks.

The present invention is generally characterized by the provision of means responding to current changes in the work gap and means responding only to one direction of the drive 'by which the electrode feed arrangement is actuated, for applying signals to a device connected on the output side of the said means, which device can respond only to the simultaneous presence of the signals and only after a predetermined time.

The protective arrangement according to the invention provides the further important advantage that it is so designed and is so adaptable that it can cooperate with any conceivable form of construction of reversible electrode feed arrangement without modifications having to be made to the latter.

Other features, objects and advantages of the invention will become apparent by reference to the following detailed description and drawing, the single figure of which depicts a preferred embodiment of the inventive protective device of arrangement for electro-erosive metalworking.

In the following there is described an electrode feed arrangement incorporating a servomotor which influences the movement of the electrode through a reduction gear. The invention is also applicable to other types of feed arrangements, for example those comprising two servomotors which act on the electrode movement through two reduction gears and a differential gear. Hydraulic and pneumatic electrode feed arrangements are also contemplated. In the latter case, instead of the servomotor or servomotors there is provided an electromagnetic setting device comprising two electric coils for the forward or backward movement of the electrodes. In all these types of electrode feed arrangements, the circuit of the protective device or. arrangement according to the invention is the same as will be hereinafter described.

After response of the protective arrangement according to the invention, which consists of simple and cheap component elements, the current supply to the working gap is interrupted and the electrical parameters, such as pulse frequency, the pulse interval ratio, the pulse amplitude and the pulse width which have been set at the pulse generator, are changed. In this way, on the one hand, the arc is interrupted and, upon restoration of the current supply, the formation of a further arc is avoided. The electrical parameters may be changed either separately or all together. This may be achieved in accordance with the working operation to be performed (roughing, finishing) and/or in accordance with the materials present as the work electrode and as the tool.

A preferred embodiment of the invention will be subsequently more fully described by way of example with reference to the drawing. Such shows the circuit arrangement of the protective device, which consists of the component parts 2, 3, 4 and 6, in combination with a pulse generator 1 and a reversible electrode feed arrangement 5 (for example motor M). This servomotor M may be thought to incorporate an armature and influences electrode movement through a suitable reduction gear, all such components for convenience in illustration being collectively designated by reference character 50. Numeral 50 may be also thought of as representing two servomotors influencing electrode movement through two reduction gears and a differential gear, or, in fact, can be considered as any other suitable electrode feed arrangement, specifically those previously mentioned, such as pneumatic or hydraulic drives having electromagnetic setting or adjustment means. Pulse generators having no storage members are known, and therefore only a part of such a pulse generator 1 is shown. Specifically, reference numerals 100 and 101 denote the electro-erosion current circuit which terminates at the electrode 106 and the workpiece 107.

The transistor TR1 is connected, as an electronic switch, to the line 100. This transistor TRl, which is only shown once, may naturally constitute a number of parallel-controlled electronic switches. Such electronic switch TR1 is actuated by the timer 109. When the working gap between the work or working electrode 106 and the workpiece '107 is sufliciently small and the transistor TRl has been brought into the conductive state, there is set up at the work gap a working spark corresponding to the voltage UB applied to the electro-erosion circuit .100, 101. During normal operation, the electrode 106 is pushed more deeply into the workpiece 107 by the reversible electrode feed arrangement 5 in accordance with the amount of material removed from such workpiece 107. if an arc is struck by impurities which have become firmly lodged in the work gap G, the reversible feed arrangement 5 is reversed and pulls the electrode 106 out of the work gap. As already mentioned in the foregoing this often produces an elongated are which welds further impurities in the working gap due to its melting effect. These welded impurities may fill at least a part of the recess in the workpiece if the arc is prolonged and may increase above the liquid level of the dielectric liquid. The arc thus burns outside the liquid level of the dielectric liquid, which may sometimes give rise to a serious fire hazard in the electroerosion machine.

Each time a normal working spark or a short-circuit exists in the working gap, or an arc jumps from the electrode -6 to the workpiece 107, there is set up across the resistor 11 a voltage drop, due to the action of which a current is passed through the conductors 200 and 201 to an electromagnetic relay 25. This relay 25, which may alternatively be constructed as an electronic switch or transductor, has in its circuit a current-limiting resistor 22, while in parallel with the relay 25 there is connected a Zener diode 23 and a capacitor 24. The Zener diode 23 maintains the voltage in the arrangement 2 at a constant value. This is important because the voltage drop across the resistor 11 fluctuates within wide limits. The capacitor 24 is provided an an integrating member, and consequently the pulsating voltage between the conductors 200 and 201 is converted into a mean value. Only when the capacitor 24 is fully charged does the relay 25 respond or pull up, so that the contacts 2.6 and 27 are connected together. It will thus be apparent from the foregoing that the relay 25 responds only when a sulficiently large current has flowed for a relatively long time across the work gap G between the electrode 106 and the workpiece 107, that is to say, even when an arc is present.

As soon as the arc has been struck in the work gap G, the servomotor M of the electrode feed arrangement 5 responds and withdraws the electrode 106 from the workpiece 107. As described in the foregoing, the arc is also pulled along. When the electrode feed arrangement 5 withdraws the electrode 106, an armature voltage is set up in such a manner that the junction or connecting point 508 is positive in relation to the junction or connecting point 509. This potential is applied to the component element 4. Connected to the connecting point 508 are a current-limiting resistor 412 and a diode 413, a resistor 415 and the base of the transistor TR2. The conductor 407 is connected to the connecting point 509'. The Zener diode 4'14 maintains the voltage at a constant value. When the motor M runs in reverse owing to the presence of an arc in the work gap G, a positive signal is set up at the capacitor 416, with the result that the base of the transistor TR2 is positive. The npn-transistor TR2 is thus rendered conductive in its collector-emitter path. A negative potential passes through the resistors 419, 420 to the base of the npn-transistor TR3, so that its collector-emitter path is blocked or rendered non-conductive. There is applied through the resistors 418, 422 to the base of the npn-transistor TR4 a negative pulse in relation to the emitter, so that the collector-emitter path of this transistor is rendered conductive. The electromagnetic relay responds or pulls up through the conductor 406, the resistor 424, the transistor TR4 and the conductor 407, and connects the contacts 43 and 44. This relay 45 may of course be an electronic switch or a transductor.

When the relay 25 has pulled up owing to the presence of an arc in the work gap G, and the relay 45 has simultaneously responded or pulled up during reverse running of the servomotor M, voltage is applied to the input of the component element 3 via the conductor 102, the contacts 21, 26, 27, 44, 43, the diode 36, the variable resistor 37, the capacitor 38 and the conductor 103. The diode 36 rectifies the alternating voltage present at the conductors 102, 103. This alternating voltage may be, for example a normal network voltage of 220 volts. The variable resistor 37, which is constructed as a potentiometer for example, and the capacitor 38 are time-determining RC members. Connected in parallel with this memher is a relay valve 39, the anode circuit of which includes an electromagnetic relay 310. As soon as the capacitor 38 has been charged, the relay valve 39 constructed as a glow discharge lamp ignites, whereby the relay 310 is pulled up or responds. The control voltage of the whole electroerosion installation is present at the conductors 104, 105.

When the normally open contact 33 is changed over from the position shown, the electro-erosion installation is de-energized. On changeover of the contact 34 from the position shown, an acoustic and/or optical warning device, denoted by reference numeral 311, is triggered. On changeover of the contact 35, an automatic device 6 is energized, which changes the electrical parameters such as, for example, the pulse repetition frequency, the pulse interval ratio, the pulse amplitude and the pulse width. At the same time, the automatic device 6 can increase the pressure in the dielectric liquid and the level of this liquid, so that the washing conditions in the working gap are improved. The automatic device 6 may be selectively set in operation upon attraction of the relay 310 as a result of appropriate construction of the contact 35 of the aforesaid relay.

It is thus apparent from the foregoing that when an arc has been present in the work gap for an adjustable or settable time (relay 25 has pulled up) and the servomotor M of the electrode feed arrangement 5 rotates in the reverse direction (relay 45 pulled up), the relay 310 pulls up after a predetermined time, which is adjustable by means of the potentiometer 37, and interrupts the voltage UB of the pulse generator through the contact 33 and the conductors 104 and 105. The are is thus immediately quenched, so that the relay 25 in the device 2 falls off. At the same time, the reverse rotation of the servomotor M is stopped. Consequently, no positive potential in relation to the connecting point 509 is present at the connecting point 508, so that the relay 45 is de-energized through the transistors TR2, TR3, TR4 and also falls off. As soon as the relay 310, which may be an electronic switch or a transductor, has fallen off, the voltage UB is restored at the pulse generator 1. The servomotor M rotates in the forward direction, i.e. the electrode 106 is again fed forwards in the direction of the work gap G, until normal working pulses are again set up. However, should an arc be reformed, owing to the impurities which have still not been washed away by the dielectric liquid, the relay 25 pulls up again, and as soon as the servomotor M iS reversed again owing to the arc, the relay 45 also begins to pull up. Consequently, the main voltage UB at the pulse generator 1 is again interrupted by the attraction of the relay 310. This cycle may be repeated a number of times, but in practice it is found that, owing to the changes of the electric parameters at the pulse generator 1 and of the washing conditions of the dielectric liquid in the working gap (no further arc can be formed when the whole installation is restarted by the relay 310.

The box denoted by reference numeral 6 contains an automatic control device for the pulse generator 1 and the circuit of the dielectric liquid. The circuit is not shown because it is known as such in erosion machines. In the following, it will be explained how the automatic control device 6 changes the electrical parameters set up at the pulse generator 1, such as the pulse frequency, the pulse interval ratio, the pulse amplitude and the pulse width, after response of the whole protective arrangement, i.e. after occurrence of an arc. The automatic control device 6 consists of known switching parts, such as a programing device, an electronic counting circuit or a stepby-step selector. The programing device may be an astable multivibrator which, according to the desired program, applies a predetermined number of pulses to the step-by-step selector or to the electronic counting circuit consisting of bistable multivibrators. In the case of the latter, the number of bistable multivibrators present is equal to the desired number of counting stages. The output of each counting stage is connected to the control input of one of the aforesaid electrical parameters in the pulse generator 1 and the conveyor pump in the circuit of the dielectric liquid. The same applies to the outputs when a step-by-step selector is employed instead of the elec tronic counting circuit. When the relay 310 in the device 3 has pulled up on occurrence of an arc in the work gap G and simultaneous reverse running of the electrode feed arrangement 5, the control device 6 is energized by the contacts 35. It is to be noted that the control device 6 need not in all circumstances be associated with the protective arrangement. It may be optionally added to the protective arrangement as an advantageous improvement.

In the following, the operation of the control device 6 in combination with the whole protective arrangement will be explained. When the relay 310 has pulled up, the programming apparatus in the control device emits a number of pulses pre-set in the programing arrangement by an operator, for example by means of a normal telephone dial. The operator can thus pre-set by means of the dial the changes required at the pulse generator 1 or in the circuit of the dielectric liquid after the occurrence of an are, so that when the relay 310 falls off (restarting of the Whole erosion installation) no further arcing can occur. Fixedy associated with each output of a counting stage within the electronic counting circuit or the step-bystep selector is a control input for the electrical parameters at the pulse generator 1 and for the liquid circuit. Depending upon the program fed in, the desired outputs are selected in turn. The potential change occurring at the selected outputs produces a change of the electrical parameters and washing conditions which have been set in the pulse generator 1 and in the liquid circuit before the response of the whole protective arrangement. The change takes place in a particular ratio in relation to the value originally set. Thus, for example, the pulse width may be reduced by 10 percent and/ or the operating voltage UB raised by a few percent and the liquid pressure may 'be raised by a few percent. Preferably, the change fed into the programing arrangement is one which affects only those control quantities which, as shown by practical experience, may greatly favor restriking of the are after restarting of the whole erosion installation, or which obviate the causes of the arc.

The protective arrangement according to the invention protects the whole electro-erosive working installation in such a manner that protracted arcs cannot give rise to any danger. This is particularly important because a number of electro-erosion installations are attended to by one operator or the installations are operated at night. The protective arrangement thus makes it possible for the electro-erosion installations to be run without supervision while optimum efiiciency is maintained in regard to removal of material.

Although in the exemplary embodiment there is shown a resistor 11 provided in the erosion circuit in the work gap G to tap the input voltage, it also would be possible to use a coil (current transformer) disposed around a part of the erosion circuit directly at the work gap for tapping such input voltage.

While there is shown and described a present preferred embodiment of the invention it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practised within the scope of the following claims.

What is claimed is:

1. In combination with an electro-erosion metalworking machine having a pulse generator and a reversible electrode feed arrangement, a protective device for interrupting arcs in the work gap defined by the electrode and workpiece of the erosion circuit, said protective device comprising means responding to current changes at the work gap, means responding only to one direction of driving of said electrode feed arrangement, and means for receiving signals from said current change-responding means and said means responding only to one direction of driving of said reversible electrode feed arrangement, said signal receiving means only being actuated upon simultaneous presence of signals from both said current change-responding means and said means responding only to one direction of driving of said reversible electrode feed arrangement and only after a predetermined time.

2. The combination according to claim 1, wherein said current change-responding means comprises a control circuit incorporating a Zener diode for maintaining constant, and a capacitor for integrating, the pulsating input voltage tapped in the erosion circuit.

3. The combination according to claim 2, further including a resistor provided in the erosion circuit in the work gap to tap the input voltage.

4. The combination according to claim 2, wherein a coil (current transformer) is disposed around a part of the erosion circuit directly at the work gap for tapping the input voltage.

5. The combination according to claim 1, wherein said electrode feed arrangement comprises a drive incorporating a servomotor with a reduction gear.

6. The combination according to claim 1, wherein said electrode feed arrangement comprises a drive having two servomotors with a differential gear.

7. The combination according to claim 1, wherein said electrode feed arrangement includes a hydraulic drive having an electromagnetic adjustment element.

8. The combination according to claim 1, wherein said electrode feed arrangement includes a pneumatic drive having an electromagnetic adjustment element.

9. The combination according to claim 5, wherein said servomotor includes an armature, said means responding to only one direction of driving of said electrode feed arrangement being connected on the input side to said armature of said servomotor and comprising a switchlng relay, a Schmitt trigger and a transistor actuating said switching relay.

10. The combination according to claim 1, Wherem said electrode feed arrangement includes a fluid control valve having an electromagnetic coil, said means responding to only one direction of driving of said electrode feed arrangement being connected on the input side to said electromagnetic coil and comprising a switching relay, a Schmitt trigger and a transistor actuating said switching relay.

11. The combination according to claim 1, wherein said electrode feed arrangement incorporates a servomotor, said means responding to only one direction of driving of said electrode feed arrangement incorporates a Zener diode for maintaining constant, and a capacitor for integrating, the armature voltage of said servomotor.

12. The combination according to claim 1, further including switch means, said signal receiving means being electrically coupled with said switch means and comprising a time-delay RC member, a switch actuated by said RC member, a switching relay in the control circuit of which there is arranged said switch, said switching relay when responding being capable of interrupting the current supply to said pulse generator, a warning device actuated by said switching relay.

13. The combination according to claim 12, further including a control device acted upon by said switching relay for selectively changing the electrical parameters at References Cited UNITED STATES PATENTS 5/ 1965 Henry. 11/1966 Livshits.

RICHARD M. WOOD, Primary Examiner.

0 R. F. STAUBLY, Assistant Examiner. 

1. IN COMBINATION WITH AN ELECTRO-EROSION METALWORKING MACHINE HAVING A PULSE GENERATOR AND A REVERSIBLE ELECTRODE FEED ARRANGEMENT, A PROTECTIVE DEVICE FOR INTERRUPTING ARCS IN THE WORK GAP DEFINED BY THE ELECTRODE AND WORKPIECE OF THE EROSION CIRCUIT, SAID PROTECTIVE DEVICE COMPRISING MEANS RESPONDING TO CURRENT CHANGES AT THE WORK GAP, MEANS RESPONDING ONLY TO ONE DIRECTION OF DRIVING OF SAID ELECTRODE FEED ARRANGEMENT, AND MEANS FOR RECIEVING SIGNALS FROM SAID CURRENT CHANGE-RESPONDING MEANS AND SAID MEANS RESPONDING ONLY TO ONE DIRECTION OF DRIVING OF SAID REVERSIBLE ELECTRODE FEED ARRANGEMENT, SAID SIGNAL RECEIVING MEANS ONLY BEING ACTUATED UPON SIMULTANEOUS PRESENCE OF SIGNALS FROM BOTH SAID CURRENT CHANGE-RESPONDING MEANS AND SAID MEANS RESPONDING ONLY TO ONE DIRECTION OF DRIVING OF SAID REVERSIBLE ELECTRODE FEED ARRANGEMENT AND ONLY AFTER A PREDETERMINED TIME. 